1. Field of the Invention
The present invention relates to an image forming apparatus including a light scanning apparatus.
2. Description of the Related Art
An electrophotographic image forming apparatus (hereinafter referred to as “image forming apparatus”) is configured to form an image on a sheet through an electrophotographic process. As the image forming apparatus, for example, there are known an electrophotographic copying machine (such as a digital copying machine), an electrophotographic printer (such as a color laser beam printer), and a facsimile machine. The image forming apparatus not only encompasses an image forming apparatus configured to form a monochrome image but also encompasses a color image forming apparatus.
A light scanning apparatus of the image forming apparatus emits a laser beam (hereinafter referred to as “light beam”), and scans a surface of a rotating photosensitive drum with the light beam, to thereby form an electrostatic latent image on the surface of the photosensitive drum. A developing device develops the electrostatic latent image using toner to form a toner image. A transferring device transfers the toner image, which is formed on the photosensitive drum, onto a sheet. A fixing device fixes the toner image onto the sheet by a thermal fixation. In this manner, an image is formed.
In general, a main body of the image forming apparatus includes a frame or side plates opposed to each other. A supporting member configured to support the light scanning apparatus is stretched over the frame or the side plates. The light scanning apparatus is supported by the supporting member in a vertical direction, and is fixed in a state of being supported by the supporting member. The light scanning apparatus (optical box), which is fixed by the supporting member, may be deformed in conformity with the flatness of the supporting member. Therefore, strict design is necessary regarding the flatness of the supporting member and the flatness of a seating surface of the light scanning apparatus, which is held in abutment against the supporting member.
There is known a method of supporting and fixing the light scanning apparatus at three seating surfaces (three points) as a method of stably supporting the light scanning apparatus by the supporting member while preventing the deformation of the light scanning apparatus, which may be caused in conformity with the flatness of the supporting member (Japanese Patent Application Laid-Open No. 2006-85058).
According to Japanese Patent Application Laid-Open No. 2006-85058, the light scanning apparatus is fixed to the supporting member at three seating surfaces. The three seating surfaces are provided at respective apexes of a substantially isosceles triangle, and are arranged in a bottom portion of the light scanning apparatus in a balanced manner in conformity with the shape of the light scanning apparatus. In Japanese Patent Application Laid-Open No. 2006-85058, the seating surfaces are provided at both horizontal end portions on one side of the light scanning apparatus. Further, the seating surface is provided at the center in a width direction on another side of the light scanning apparatus. The seating surfaces are provided on respective fixing portions provided on the light scanning apparatus. Surfaces opposed to the seating surfaces of the fixing portions in the vertical direction are pressed by plate springs, respectively. With this, the light scanning apparatus is fixed to the supporting member.
The image forming apparatus includes driving sources configured to drive the developing device, the photosensitive drum, and the like. Vibrations of various frequencies, which are generated in the driving sources, are transmitted to the light scanning apparatus via the side plates of the image forming apparatus and the supporting member.
When the three seating surfaces are used as in Japanese Patent Application Laid-Open No. 2006-85058, torsional vibration is generated on the other side of the light scanning apparatus due to the vibrations generated in the driving sources. The torsional vibration causes a rotation about a central axis extending from the other side to the one side of the light scanning apparatus and passing through the seating surface at the center on the other side of the light scanning apparatus. However, the one side of the light scanning apparatus is fixed at the seating surfaces of both the horizontal end portions, and hence the torsional vibration that causes a rotation about the central axis is not generated on the one side of the light scanning apparatus. When the torsional vibration is generated on the other side of the light scanning apparatus due to the vibrations generated in the driving sources, a light beam, which is emitted from the light scanning apparatus, scans the photosensitive drum in a vibrating manner. Therefore, the photosensitive drum is irradiated with the light beam with unevenness in irradiation position. The unevenness of the irradiation positions causes an image defect in a stripe shape, which is called “uneven pitch”.
The optical box of the light scanning apparatus of such a type that the light beams, which are emitted from a plurality of light sources of the single light scanning apparatus, respectively, are deflected by a single rotary polygon mirror at the same time may have a rectangular parallelepiped shape or a polygonal (quadrangular or more) box shape, which is similar to the rectangular parallelepiped shape, due to the large number of mirrors and lenses and the arrangement thereof. Therefore, when such a light scanning apparatus is fixed to the supporting member at the three seating surfaces, the torsional vibration is generated conspicuously, in particular.